Surge protection devices are commonly used to protect electronic equipment from transient voltage surges or spikes in AC line power. Such spikes or surges may result from lighting strikes to the power grid, switching transients, and other anomalies within the power grid. Spikes or surges in voltage can be damaging to sensitive electronic equipment. Accordingly, consumers often use a surge protection circuit like a power strip to connect to a household AC power outlet, and plug equipment into AC outlets on the power strip.
Many surge protection devices employ metal-oxide varistors (MOVs) to absorb voltage spikes or surges above a threshold value. MOVs work by absorbing voltage spikes above a preset level and converting the excess energy into heat. The heat is dissipated by the MOV. The capacity of the MOV to absorb voltage spikes without excessive heating and consequent rupturing is limited. Repeated voltage spikes have a cumulative deleterious effect on the capacity of the MOV to absorb additional spikes without overheating. When an MOV overheats beyond its tolerance, it can rupture and burst.
To prevent the MOV from rupturing, electrical safety standards require the use of a corresponding thermal fuse designed to disconnect the MOV in the event that its temperature reaches a critical value. Accordingly, the thermal fuse may remove the MOV from the surge protection circuit to prevent it from bursting.
In many cases, surge protection circuits are designed such that when the thermal fuse opens the surge protection circuit is disabled, but AC power is still provided at the outputs. The failure of the thermal fuse may trigger an LED or may de-power an LED so as to indicate that a thermal fuse within the surge protection circuit has failed. In other words, on failure of the thermal fuse the surge protection feature is disabled and it becomes incumbent upon the user to disconnect electrical devices from the power strip and to repair or replace the power strip. Such power strips may be classified as “fail-closed” devices.
Line conditioner units that incorporate surge protection circuits are often used to protect valuable and sensitive electronic equipment, such as high-end home theater and stereo equipment. The line conditioner units provide appropriate filtering, smoothing, isolation, and other power conditioning features in addition to surge protection. As a result, such units are more expensive than simple power strips and consumers cannot be expected to replace the unit in the event that a component within a surge protection circuit fails.
It would be advantageous to provide for improved surge protection and to provide for an improved line conditioner unit having surge protection circuitry.